The chances Greece will leave the euro this year now look greater than ever, UK bookmaker William Hills said on Wednesday.

The latest odds of 5/4 imply a roughly 45 percent chance of so-called "Grexit" before the end of the year, compared with odds of 6/4, or 40 percent probability, earlier this week.

It is offering odds of 4/7 that Greece will still be in the Eurozone on January 1 next year, implying a probability of around 64 percent.

This comes as the Greek debt crisis enters its most critical phase yet. Prime Minister Alexis Tsipras must present proposals that are acceptable to Athens' international creditors by Sunday or else funding will be cut off and Greece will crash out of the currency bloc.

"The pendulum has swung again and punters are now betting that Greece will begin or complete the process of leaving the euro zone this year," said William Hill's spokesman Graham Sharpe.

Several banks, including JP Morgan and Barclays, now say 'Grexit' is their base case scenario following a referendum on Sunday at which Greek voters overwhelmingly rejected a bailout that carried stringent austerity measures.

On Monday, UK bookmaker Ladbrokes said more money was being placed on Greece staying in the euro after controversial finance minister Yanis Varoufakis resigned.

It offered odds of 2/5, implying a roughly 70 percent chance, of Greece remaining in the 19-nation bloc.